1. Field of the Invention
The present invention relates to hoisting attachments and, more particularly, to anchor members which are permanently imbedded in prefabricated concrete building components for the attachment thereto of hoisting means capable of lifting and transporting the building components.
2. Description of the Prior Art
The use of prefabricated building components of reinforced cast concrete has become widely accepted in the construction industry. For economic reasons, the preferred way of producing and installing these building components is to have a centralized place of production, a prefabrication factory, where the components are produced, and to then transport the finished building components to the construction site. For the production of large concrete panels and frames, such as wall members, for example, such a prefabrication factory would be equipped with a hydraulically tiltable casting table which allows for the wall members to be lifted from the table in the upright orientation in which the prefab members are then transported to the construction site. The tiltability of the casting table greatly simplifies the hoisting problems, making it possible to use simple traction anchors in the upper narrow face of the wall member. This means that, immediately after production, the wall member is placed in an upright or near upright orientation in which it can be lifted and transported in the same manner as a prefabricated beam, for example.
There are many situations, however, in which a tiltable casting table is unavailable, especially when the wall members are to be cast at the construction site itself. Under these circumstances, it becomes necessary to tilt the finished wall member into the upright position by raising it on that edge which will later be the upper edge of the member, using suspension-type lifting equipment, such as a construction crane, for example. It is, of course, desirable to utilize for this purpose the same anchor elements in the narrow face at the upper edge which also serve for the hoisting and handling of the wall member, after it has been tilted up.
A major problem with this approach lies in the requirement that the anchor elements be set below the level of the edge face, inside a suitable recess, which is open in the direction perpendicular to that face. This means that, in the horizontal position of the prefabricated member, the attachment gear, after connection to the anchor element, will pull at a right angle to the normal direction of pull, tending to break away the narrow concrete ledge between the anchor element recess and the edge above it, or to break out the embedded anchor element altogether. Naturally, this problem is particularly acute in the case of thin wall members, where the ledges in question are very narrow.
While it is possible to prevent the lateral breakout of the anchor elements by means of suitable reinforcing rods or legs which increase the bearing surface of the anchor element appropriately, it is very difficult to arrange the hoisting attachment in such a way that the shackle part of the equipment which is connected to the embedded anchor element be pivotable in the sense toward the second anchor, but not in the direction of tilt-up pull.
A hoisting attachment of the type mentioned is disclosed in U.S. Pat. No. 3,883,170, the attachment consisting of an anchor element which is permanently embedded in the concrete member and a clasp-like hoisting shackle with a slot and an arcuate locking bolt which cooperates with an exposed eye portion of the anchor member to produce a secure connection. Because it is in most cases necessary that the anchor element be recessed below the surface of the prefabricated member, the latter has to have a suitable elongated recess surrounding the exposed eye portion of the anchor element, the recess having a shape which closely matches the ring-shape of the body of the hoisting shackle.
The exposed eye portion of the anchor element, when engaged in the attachment slot of the hoisting shackle and locked in place by its arcuate locking bolt, prevents the hoisting shackle from pivoting in the longitudinal sense of the concrete member, but allows pivoting movements in the transverse or lateral sense, about an axis which is defined by the hole in the anchor element eye portion and the engaged locking bolt. This lateral pivotability of the hoisting shackle is of no consequence, when the pull of the hoisting harness is in a plane which coincides with the longitudinal axis of the embedded anchor element. However, in the case of a horizontally oriented wall plate, for example, the anchor element, or elements, are initially embedded in a horizontal orientation, their exposed eye portions and access recesses being arranged in the vertical narrow face of the member. This means that the hoisting shackle, when attached to the anchor element, extends likewise horizontally away from the vertical face of the wall member.
It should be evident from this description that, if the hoisting harness, in an attempt to tilt the wall member into an upright orientation, exerts an upward pull on the hoisting shackle, the latter will pivot under this force until one side of the shackle body engages the upper side wall of the recess in the concrete member. In many cases, especially when the wall member is comparatively thin, this situation leads to the breakaway of the concrete against which the shackle body is being pressed.
While the prior art solution proposes an embodiment in which the eye portion of the anchor element has a pointed outline with an obtuse crest angle engaging an attachment slot of the hoisting shackle with an approximately matching angled bottom end, this attachment configuration, though seemingly restricting the pivotability of the hoisting shackle in the lateral sense, is unsuitable for tilt-up operations. The two faces on the exposed eye portion of the anchor element and the two faces at the bottom of the attachment slot of the shackle body are so close to a tangent line on the pivot center, i.e. the hole of the eye portion, that the creation of a pivotability-restricting force of sufficient magnitude is impossible.
It has therefore already been suggested to utilize special tilt-up anchor elements in combination with the above-described hoisting shackle, each anchor element having welded to its exposed eye portion a cradle-like collar piece which wraps around the shackle body on opposite sides of its attachment slot, thereby restricting its pivotability as required for a tilt-up operation. This composite anchor element is complex in structure and comparatively costly to manufacture.
Another prior art hoisting attachment features a permanently embedded anchor element which has a ball-shaped exposed extremity which is being engaged by a hoisting shackle with a matching cavity and a locking mechanism engaging the ball-shaped end. Here again, the pivotability provided by the ball-shaped exposed portion of the anchor element allows for the hoisting shackle to pivot against the concrete forming the wall of the recess, thereby tending to break away that concrete, when the hoisting attachment is used in a tilt-up mode of operation.
In order to solve this problem, it has therefore been suggested that a specially modified hoisting shackle be used which includes a plate which, by supporting itself against the outer surface of the concrete wall member, prevents the hoisting shackle from pivoting into contact with the concrete. While this solution has the disadvantage of covering the recess around the anchor elements, thus preventing visual inspection of the attachment mechanism, it also has the disadvantage of greatly increasing the traction on the attachment mechanism itself, as a result of the pressure of the special shackle plate against the concrete surface.
Still another prior art hoisting attachment features an anchor element with a male thread on its exposed length portion, which is to be engaged by a hoisting shackle with a matching female thread. Here, too, special provisions have to be made to prevent the surrounding concrete from breaking away under excessive stress. The solution proposed in this case is similar to the one previously described, namely a special plate added to the hoisting shackle which bears against the concrete member from the outside.
It has also been suggested that such an exterior supporting plate, preferably U-shaped, be attached to the anchor element itself. However, this approach is very costly, because it means that each anchor element has to be equipped with such a plate which cannot be recovered after use.
Lastly, there is also in use a hoisting attachment which features a tubular anchor element with a free cavity underneath the embedded extremity of the element. The cooperating hoisting shackle has a matching member which reaches through the hollow anchor element, locking itself against the latter by means of a locking head with radially movable members occupying the cavity behind the anchor element. While this approach does not require that a portion of the anchor element be exposed inside a surrounding recess in the concrete member, thereby eliminating the difficulties caused by a recess in connection with tilt-up operations, the necessity of a cavity behind the embedded extremity of the tubular anchor element involves offsetting disadvantages, especially as regards the impossibility of a visual inspection of the locking action of the hoisting shackle and the risk that gravel and/or water may be found in the cavity, the latter being particularly dangerous at freezing temperatures.